Non-volatile memories are ubiquitous. They are used in digital cameras, cell phones, music players, computers, and many other devices where non-volatile retention with rapid reading is of interest. Semiconductor non-volatile memories provide speeds which while slower than of SRAMs, are faster than possible through other means, e.g., magnetic disks.
The most common forms of non-volatile memory are various manifestations of electrically erasable and programmable memory structures employing a floating gate region in which charge is stored. Many new manifestations of this structure use few electrons, single electrons, and defects to lower the power and to allow scaling to dimensions lower than those possible with continuous floating gate regions. Memories have also employed defects and storage on the back of a silicon channel, thus allowing simultaneous transistor and memory capabilities.
Ultimately, as all these approaches employ electrons and holes for storage, the scalability is constrained by the number of electrons and the reliability issues arising from leakage of carriers and generation of defects during injection and extraction. A reliable low power memory device that can be scaled to transistor's ultimate limit near 10 nm and that can have high endurance and high speed is highly desirable.
In order to achieve memory function, one needs two quasi-stable states. In electronic silicon non-volatile memories, the quasi-stable states are achieved by storing charge on a continuous or discrete floating gate region surrounded barrier regions made of silicon dioxide and/or other dielectrics that prevent leakage of stored charge. The presence or absence of this charge is measured through the operation of a transistor whose threshold voltage is affected by the stored charge. A non-volatile memory is usually implemented with two stable states, but more are possible depending on the ability to achieve distinction between reproducible stored charge number, as also in the location of the charge, e.g., between the source-end region and drain-end region of a transistor. Thus, these nonvolatile memories depend on electron transport phenomena—both in the transistor which is the reading and writing medium and the floating gate region.
There are additional approaches. One group of approaches aims to achieve non-volatile memory where a transistor is coupled to an additional passive element-a ferroelectric element where polarization is changed, or a phase change element where the resistance of a conducting element is changed. These elements operate by changing the conductivity in the high to low potential path of a cell.
Many new manifestations of the floating gate structure based memories use few electrons, single electrons, and defects to lower the power and to allow scaling of dimensions to dimensions lower than those possible with continuous floating gate regions. The physical character of the problem of finding a useful replacement or augmenting the current approach to information processing beyond the end of scaling of CMOS is constrained by:                Size-domain: any state property employed must be sustainable and insensitive to the environment and the interface in the 1-10 nm dimension range,        Energy-domain: any state change employed must have a strong energy minimum, i.e. have large barrier energies (>>kT or competing processes'energy scale) to suppress probability of a disturb and yet require low enough energy so that property is useful at large densities of integration,        Time-domain: the state changes must occur at time-scales that support circuit architectures with real-time use and the state property employed must have coherence times larger than computation time,        Signal Sensitivity and Strength: the state property must remain relatively insensitive to the environment and be strong enough to be easily detected and recoverable in any implementation.        
All possible approaches: employing properties of charge, spin, magnetic flux quantum, photon energy, polarization, entanglement, etc. and their implementation in semiconductors, magnetic materials, ferroelectric materials, ferromagnetic materials, optical materials, organic materials—molecules e.g. have limitations that arise from size, energy, time and signal strength. The dominance of charge-based approaches (and of voltage and current as signal), such as in the transistor and the memories, has its foundation in long coherence time and a high signal strength with desired time and energy scales at useful dimensions. This property holds in a variety of materials (semiconducting inorganic and organic) because of efficient transport and control of field-effect. The failure of this approach in the 1-10 nm size range arises from the loss of dominance of the transport mechanism to tunneling, the loss of reproducibility because of the loss of collective effects making the device sensitive to the environment, and the consequences of energy-time interaction whose one manifestation is power-dissipation.
The loss of collective effects through size scaling is a common change in character of all alternatives. A5 nm×5 nm×5 nm volume can potentially hold ˜10's of thousands of atoms/nuclei and electrons in a metallic system, and a larger number of bound electrons. The number of electrons potentially employable in semiconducting inorganic and organic systems is significantly smaller, even reducing to single digits in presence of classical single electron effects. Similar arguments also hold for approaches based on photons. Magnetism, ferroelectricity, and metallic conduction are examples of state properties that continue to benefit from large collective effect at the smallest scale, although inevitably these too must overcome surface and interface induced competition (paramagnetism, surface scattering, etc.).